1. Field of the Invention
The present invention relates to a communication apparatus and a display terminal to realize a function of sharing a screen of an application, for example, between computers.
2. Description of the Related Art
There is a computing system in which for the purpose of improving usability, a terminal apparatus including a minimum of input/output interfaces is placed on the user side and complex computing is performed on a main apparatus located in a remote place.
For example, a system configuration in which screen information on a main apparatus (such as a personal computer or a server computer) is projected onto a remote display terminal via a network is already disclosed in U.S. Pat. No. 6,784,855 (hereinafter referred to as “Reference 1”).
In this system, input information (such as a pen-based input by a digitizer) from the display terminal is similarly sent to the main apparatus via the network, and actual application program processing is executed by the main apparatus. Thereafter, a result of the execution and screen update information are transferred to the display terminal via the network. The terminal apparatus executes output (drawing) processing by the received screen update information.
On the other hand, as a technique for efficiently transmitting the screen information from the main apparatus on the remote network to the terminal apparatus, VNC (Virtual Network Computing) is known.
In this VNC, when an update of a screen is detected, a value of read pixel information is compared with a value of pixel information last transmitted to the display terminal, and an updated screen area changed from the last time is determined.
Further, by subjecting the updated screen area to still image compression and then transmitting only difference information on the compressed screen to the display terminal, communication band consumption can be suppressed.
Accordingly, the amount of screen information to be sent increases when a screen change such as a movement of a window is large, whereas the amount of screen information to be sent decreases when a screen change is small.
Incidentally, by performing MIMO (Multiple Input Multiple Output) transmission when the display terminal performs communication by wireless LAN, an effect of expanding an available distance range, an effect of improving response when large-sized data is transmitted, and so on are conceivable.
MIMO is a technique of simultaneously sending/receiving signals from plural antennas using the same frequency channel, but compared to SISO (Single Input Single Output: a conventional sending/receiving method by one antenna), the same number of sending/receiving circuits as that of antennas are needed, which causes a problem that power consumption increases in proportion to the number of antennas.
Hence, in recent years, a mechanism to reduce power consumption regarding MIMO is proposed, for example, in JP-A 2006-42075 (KOKAI) (hereinafter referred to as “Reference 2”).
In the case of a technique in reference 2, during standby when frame exchange is not performed, a receiving terminal operates as a SISO one by activating only one of plural receiving systems, whereas at the time of frame exchange, it performs MIMO receiving processing by supplying power to the plural receiving systems.
Incidentally, among applications to execute processing by a computer, there are presentation software, text creating software, and so on, and in addition applications using a network including video streaming playback software, voice communication software, file transfer software, and so on, and the characteristic of traffic flowing over the network variously changes.
In the case of the application such as the video streaming playback software or the voice communication software which requires responsiveness, a data packet is generated in a given cycle and flows over the network.
In the case of the application such as the file transfer software which can tolerate transmission delay to a certain extent even if it is large, a large amount of traffic is generated in a burst manner simultaneously with the start of file transfer.
In the system in which the screen information is sent from the main apparatus on the remote network to the terminal apparatus and received and displayed by the terminal apparatus, for example, a movement of a cursor by a mouse manipulation is continuous small movements.
Therefore, the size of data transferred from the main apparatus to the terminal apparatus is small, and in contrast, if an event such as a new appearance, disappearance, or movement of the window occurs, the amount of a changed difference accompanying a screen update is large, so that the size of transferred data also becomes large.
Accordingly, a screen transfer system via such a network is characterized in that traffic is generated in a burst manner and responsiveness is required.
For example, let's assume a case where a terminal apparatus which includes a display (for example, a liquid crystal display panel or a projector projection unit) displaying screen information and a MIMO wireless communication processing unit receives screen information on a main apparatus on a remote network via the wireless communication processing unit and displays it.
In this case, in MIMO, compared to SISO, the power consumption increases in proportion to the number of antennas, and besides, a synchronous signal for a high throughput terminal in addition to a synchronous signal for a legacy terminal is needed in a physical layer.
Hence, there is a problem that when small-sized data is sent, the proportion of a fixed-length physical header portion of a transmitted physical frame becomes larger (this is called transmittion overhead), so that the transmitting efficiency in MIMO lowers compared to that in SISO.
Namely, if screen update information from the main apparatus is continuous small-sized one such as a movement of cursor by mouse, and MIMO transmission is performed, not only power consumption but also overhead at the time of transmission becomes large, which can be said to be inefficient.
In the case of the above technique in reference 2, during a standby state where frame exchange is not performed, power is supplied to only one receiving system out of plural receiving systems, and every time frame exchange is performed, power is supplied to the plural receiving systems to enable sending/receiving of the physical frame by MIMO, but when a data frame is exchanged, plural antennas are always activated in the receiving terminal.
Namely, there is a problem that also when such small updated screen information as does not need sending at a high physical transmission speed by MIMO is received, the wireless communication processing unit on the receiving terminal side is operated as the MIMO one, so that unnecessary power consumption is inevitable.